[Jeff Lange]

The differential is not an R200, it is a Toyota unit. In the past referred to as the G-series (8.0") it is now referred to as the F20SX/F20TX/FD20A/FD20AT (205mm) unit. The same basic differential was used in many Toyota cars and trucks, notably the 86-92 Supra, 01-05 IS300 (with the same Torsen T2 LSD that the Zx6 gets), and the 06-15 IS250. The older models usually have a different outer case (pumpkin), however the internals are still near identical. The newer models like the IS300/IS250 have the same case and internals as the Zx6 in the 86/FR-S/BRZ.

There are many ratios available for these cars, and quite a few part numbers out there from Toyota, even for the same ratios (ring & pinion sets typically include the pinion seals, stake nut, ring gear bolts and retainers -- if applicable). Using older Supra ring and pinion sets will require you to get a different pinion seal, as the housing is different.

Still, here is a partial list of some of the ratios available from Toyota, along with some of their part numbers that will work in the Zx6:

3.58:1 - 41201-80125
3.73:1 - 41201-80050
3.91:1 - 41201-80052
4.10:1 - 41201-19605
4.30:1 - 41201-29606 (Included pinion seal will not fit)
4.56:1 - 41201-19557 (Included pinion seal will not fit)
4.88:1 - 41201-80183 (Included pinion seal will likely not fit)
5.13:1 - 41201-80300 (Included pinion seal will likely not fit)

As for the TL70 (Aisin AZ6) Transmission, it is very similar to the Toyota-designated J160 transmission from the Altezza RS200, but the ratios are actually the same as the AZ6 used in the Silvia S15, Nissan-designation FS6R92A. The TL70 in the Zx6 is a newly developed version of the AZ6, though it shares most of its design with predecessors. While most of the gear and synchro setup between the TL70/J160/FS6R92A are all the same, the external cases vary quite a bit, and the TL70 has a different shift pattern, placing reverse beside 1st instead of beside 6th. That gear pattern difference obviously necessitated considerable revisions to the shift linkage internally. The TL70 in the Zx6 also has a larger case, allowing for a higher gear oil capacity (1.8L for the FS6R92A, 2.2L for the TL70).

The TL70 shares a similar style case to the J160, both utilizing remote-mount shifters, while the FS6R92A had the shifter mounted directly on the tailshaft housing.
The TL70 shares gear ratios with the FS6R92A, while the J160 has a taller 1st gear and a shorter 6th gear, the TL70 and FS6R92A have a shorter 1st gear and a taller 6th gear (selected presumably for better fuel mileage).

EDIT (04/30/2016): Updated some information, added a couple more available ring and pinion sets.

Jeff